javascript - How to pick an indexed value from a knockout array -

I want to find an array available, with a pointer variable 'ID', this is snippet from my code:

  function items (id, category, name, selected) {var self = this; Self.id = ko.observable (ID); Self.Category = ko.observable (category); Self. Name = Co.obswable (name); Self. Selected = Ko.SourceVoice (selected); } ViewModel () {var itself = this; Self.availableItems = ko.observableArray (); Self.init = function () {self.availableItems.push (New item (1, "A", "Hello", Incorrect)); Self.availableItems.push (new item (2, "A", "hi", true)); Self.availableItems.push (new item (3, "A", "bye", wrong)); } Self.id = ko.observable (1); } Var viewModel = new ViewModel (); Ko.applyBindings (ViewModel); ViewModel.init ();  

How does the following HTML line work?

  indexed line: & lt; Span data-bind = "text: $ root.availableItems () [$ root.id ()] and selected ()" gt; & Lt; / Span & gt;  
 
 Because you must start your visual model before bonding it. 
 
 This will work: 
 
  var viewModel = new ViewModel (); ViewModel.init (); Ko.applyBindings (ViewModel);  
 
 When you call, Knockout is trying to parse and apply your binding. But your array of achievements available till then is still empty, thus you are getting an exception.